Hmm, just looking at GDAX's API, you aren't able to tell who's making an order. You don't know if it's one account with ten orders, or ten accounts with one each. So I don't see why multiple accounts would be an advantage.
You could do this by just placing orders, but I guess in some instances it would be beneficial to be able to hold positions in both directions.
EDIT: Nvm, the flag just specifies what the behavior is in the event that the system encounters a self-trade: decrement and cancel, cancel resting, cancel incoming, cancel both.